1. Field of the Invention
The present invention relates to gamma correction devices in image capturing apparatuses and, in particular, to a gamma correction device in an image capturing apparatus for correcting a video signal from an image capturing element on the basis of at least one correction curve having a predetermined input-output characteristic.
2. Description of the Related Art
In movies, traditionally, film-based cameras have been used for shooting in most cases. In order to view actual images captured with a film-based camera, it is necessary to develop the captured film and project the developed film by passing light therethrough.
In recent years, digital-based movies captured with digital video cameras have emerged, replacing film-based movies. Using the digital video cameras permits recorded images to be read immediately and to be viewed on monitors at shooting sites. In general, the monitors are optimized by curves complying with the ITU-709 standard (hereinafter referred to as an ITU-709 characteristic curve).
In digital video cameras, generally, signals having 600% or higher dynamic range output from image capturing elements consisting of charge coupled devices (CCDs) are compressed into signals having 200% to 250% dynamic range by pre-knee correction. The compressed signals are subjected to analog-to-digital conversion and 10-bit assignment. Because of 1 Vp-p constraints, the real video signals are compressed in the digital video cameras having CCDs from 600% or higher dynamic range to about 100% to 120% and then subjected to gamma correction (see, for example, Japanese Unexamined Patent Application Publication No. 5-336406).
However, since a 100% or higher dynamic range of the video signals output from the image capturing elements of the digital video cameras is significantly compressed by pre-knee correction, the gamma correction based on the conventional ITU-709 characteristic curve results in output video signals including hardly any information contained in the 100% or higher dynamic range. As a result, digital-based images captured with digital video cameras differ from film-based images captured with film-based cameras and having wide dynamic range. Moviemakers notice the difference in shades of gray and color tone between the film-based images and the digital-based images, and thus determine that the digital-based images cannot provide shades of gray and color tone to a quality equivalent to the film-based images. As a consequence, moviemaking by the digital video cameras has not yet been very popular.
Applying a logarithmic curve by which a signal is corrected so as to conform to film properties as a correction curve permits the images to include information contained in a range exceeding 100%. In this case, the slope of the rising logarithmic curve at the origin (where a level of an input signal is zero) is significantly steep, compared to applying the ITU-709 characteristic curve (having the slope of approximately 4.5 at the origin) as the correction curve. Using a conventional monitor optimized on the basis of the ITU-709 characteristic curve causes images corrected by the logarithmic curve to have a significantly isolated black component and thus the displayed images are different from the real images. Since monitors optimized on the basis of the logarithmic curve would incur high cost, it has been desired that images with a quality equivalent to film-based images be displayed even on conventional monitors.